Question

A solution X contains 30 mole percent of A and is in equilibrium with its vapour that contains 40 mole percent of B. The ratio of vapour pressure of pure A and B will be

• A. 2 : 7
• B. 7 : 2
• C. 3 : 4
• D. 4 : 3

Answer 1

The correct option is A 2 : 7

From Raoult's law
.. PA = (P*A) x (χA)
.. PB = (P*B) x (χB).. .. .. . χB is mole fraction B in LIQUID phase
and from Daltons law
.. PA = yA x Ptotal
.. PB = yB x Ptotal.. .. ... ...yB is mole fraction B in VAPOR phase
subbing
.. yA x Pt = (P*A) x (χA)
.. yB x Pt = (P*B) x (χB)
dividing
.. (yA / yB) x (Pt / Pt) = (P*A / P*B) x (χA / χB)
simplifying and rearranging
.. (P*A / P*B) = (yA / yB) x ((χB / χA)
and finally
.. (P*A / P*B) = (60 / 40) x (70 / 30) = 3.5From Raoult's law
.. PA = (P*A) x (χA)
.. PB = (P*B) x (χB).. .. .. . χB is mole fraction B in LIQUID phase
and from Daltons law
.. PA = yA x Ptotal
.. PB = yB x Ptotal.. .. ... ...yB is mole fraction B in VAPOR phase
subbing
.. yA x Pt = (P*A) x (χA)
.. yB x Pt = (P*B) x (χB)
dividing
.. (yA / yB) x (Pt / Pt) = (P*A / P*B) x (χA / χB)
simplifying and rearranging
.. (P*A / P*B) = (yA / yB) x ((χB / χA)
and finally
.. (P*A / P*B) = (60 / 40) x (70 / 30) = 3.5From Raoult's law
.. PA = (P*A) x (χA)
.. PB = (P*B) x (χB).. .. .. . χB is mole fraction B in LIQUID phase
and from Daltons law
.. PA = yA x Ptotal
.. PB = yB x Ptotal.. .. ... ...yB is mole fraction B in VAPOR phase
subbing
.. yA x Pt = (P*A) x (χA)
.. yB x Pt = (P*B) x (χB)
dividing
.. (yA / yB) x (Pt / Pt) = (P*A / P*B) x (χA / χB)
simplifying and rearranging
.. (P*A / P*B) = (yA / yB) x ((χB / χA)
and finally
.. (P*A / P*B) = (60 / 40) x (70 / 30) = 3.5